1. Field of the Invention
The present invention relates to a synchronization circuit and a synchronization method in which a packet is detected using a preamble of a packet received from a communication partner and synchronization processing is performed. The invention also relates to a wireless communication device and a wireless communication method in which a reception operation is performed on the basis of a synchronization result using the packet preamble, and a computer program. More particularly, the invention relates to a synchronization circuit and a synchronization method, a wireless communication device and a wireless communication method and a computer program that detect exact reception timing using the preamble of a packet in a receiver with a plurality of reception branches.
More particularly, the invention relates to a synchronization circuit and a synchronization method, a wireless communication device and a wireless communication method and a computer program that detect exact reception timing without increases in circuit size and power consumption for the detection of the reception timing. More particularly, the invention relates to a synchronization circuit and a synchronization method, a wireless communication device and a wireless communication method and a computer program that detect exact reception timing without using crosscorrelation for the packet preamble.
2. Description of the Related Art
Wireless networks are now attracting attention as novel communication systems to replace wired communication systems. Exemplary standards for wireless networks may include I institute of Electrical and Electronics Engineers (IEEE) 802.11 and IEEE802.15. IEEE802.11a/g is a standard of wireless LANs employing orthogonal frequency division multiplexing (OFDM) modulation, which is a multi-carrier system.
In a wireless communication system with OFDM modulation, transmitted data is subject to serial-parallel conversion and inverse discrete fast Fourier transform (IFFT) at a transmitter side to collectively modulate orthogonal subcarriers. At the transmitter side, a training signal used for synchronization called a “preamble” is added in bursts to heads of modulation signals having an IFFT-processed frame structure, which are then transmitted. At a receiver side, normalization (setup of automatic gain control (AGC) of received signal electric power, frequency offset correction, detection of reception timing and other processes are conducted using the received preamble. A FFT window is provided based on the detected timing and a discrete fast Fourier transform (FFT) operation is performed.
Detection accuracy of the reception timing has significant influence on reception performance. In the OFDM modulation system, it is necessary to optimize timing of performing FFT operations on received symbols. Mistiming in FFT operations may cause intersymbol interference and symbol rotation, which may result in impaired reception performance.
The reception timing is detected using the training signal, i.e., the preamble, transmitted in bursts using the packet head. A receiver can detect the reception timing through autocorrelation or crosscorrelation using the preamble based on the time when a correlation result exceeds a certain threshold.
Autocorrelation is a process for finding a correlation between repeated signals included in the preamble. Crosscorrelation is a process for finding correlation between a known data string and an input data string. Autocorrelation is useful even in the presence of reflection or fading, but may disadvantageously correlate data and even noise other than the preamble. Crosscorrelation does not detect correlations with noise or unrelated data, but its correlation peak may become smaller upon changes in received waveforms due to large variation in reception frequency, reflection or fading (for example, see Japanese Unexamined Patent Application Publication (JP-A) No. 2003-69546, paragraph 0007).
A method of extracting exact reception timing has been proposed in which rough reception timing (i.e., temporary reception timing) is first determined through autocorrelation in with the first half of the preamble and then reception timing is determined through crosscorrelation in a time window calculated based mainly on the temporary reception timing in the second half of the preamble (for example, see JP-A-2003-69546, paragraph 0007).
FIG. 15 illustrates a frame format of IEEE802.11a/g. FIG. 16 illustrates a preamble configuration provided by IEEE802.11a/g. As shown in the drawings, a short training field (STF) of 8.0 microseconds and a long training field (LTF) of 8.0 microseconds are added to the heads. In the short training field, short preambles t1 to t10 including short training sequences (STS) are sent in bursts, i.e., repeatedly sent 10 times. In the long training field, after a 1.6 microseconds of guard interval GI2, long preambles T1 and T2 including long training sequences (LTS) are sent twice.
The receiver may be autocorrelated between known training sequences STS usually included repeatedly in the STF and may determine a peak position of (the square of) the absolute value of autocorrelation as temporary reception timing. Autocorrelation may be calculated through, for example, cumulative addition or a moving average of a complex conjugate multiplication result of a received signal and a delayed signal received before the received signal by one repeating cycle.
In the subsequent LTF, the receiver may extract exact reception timing through crosscorrelation in time window based mainly on the temporary reception timing. As shown in FIG. 16, in the LTF, a known signal LTS of 3.2 microseconds is repeated 2.5 times during transmission. That is, the LTF configuration includes 3.2 microseconds×2.5 times=8 microseconds. The receiver then keeps a LTF table with a length of 3.2 microseconds and calculates crosscorrelation between this known LTF table and the temporary reception timing which is considered to be the LTF of the received signal.
In this manner, the cross correlation peak value as shown in FIG. 17 can be detected. Since the known signal LTS of 3.2 microseconds is repeated 2.5 times in the LTF, two peaks should be formed in a precise sense. Rough synchronization may be made in advance with the STF, a crosscorrelation circuit that detects exact timing may be provided so as to find only the first peak of the head of the LTF. A subsequent demodulation process and a FFT process may be conducted using the synchronization points determined on the basis of the peak position shown in FIG. 17.
Although the IEEE802.11a/g standard supports a modulating system which achieves a transmission speed of 54 Mbps at the maximum, there is an increasing demand for a higher bit rate next-generation wireless LAN standard. IEEE802.11n, which is an extended standard of IEEE802.11, employs a multiple input multiple output (OFDM_MIMO) communication system which performs transmission beam forming in accordance with channel characteristics in a multiantenna system.
In a MIMO receiver with multiple reception branches, an LTF crosscorrelation circuit may be mounted for each reception branch in a synchronizing circuit for performing exact reception timing. In this system, for example, the LTF cross correlation circuits for three branches are mounted and representative synchronization timing is selected from synchronization timing detected at each branch. However, the circuit for calculating crosscorrelation values is significantly large in scale, thereby increasing the manufacturing cost of the system and power consumption.
Alternatively, the receiver may only detect the synchronization timing by using the STF and detection of exact synchronization timing by the LTF may be omitted. It is clear, however, that accuracy in synchronization timing is low and reception performance may be impaired by inter symbol interference or symbol rotation.
It is desirable to provide an improved synchronization circuit and a synchronization method in which a packet is detected using a packet preamble received from a communication partner and synchronization processing is performed. It is also desirable to provide an improved wireless communication device and a wireless communication method and a computer program in which a packet is received based on a synchronization result using the packet preamble.
It is further desirable to provide an improved synchronization circuit and a synchronization method, a wireless communication device and a wireless communication method and a computer program for detecting exact reception timing using the packet preamble in a receiver with multiple reception branches.
It is yet further desirable to provide an improved synchronization circuit and a synchronization method, a wireless communication device and a wireless communication method and a computer program for detection of exact reception timing without increase in circuit size and power consumption for the detection of the reception timing.
It is still further desirable to provide an improved synchronization circuit and a synchronization method, a wireless communication device and a wireless communication method and a computer program for detection of exact reception timing without using crosscorrelation for the packet preamble.